A person who is unwilling participate in what he perceives to be risky, embarrassing, dangerous or foolhardy activities; the term is usually used to denigrate such individuals by those who do participate in the same activities.

Similar usage sometimes to 'coward', 'sissy' or 'momma's boy'.

"Don't be such a jamtart, Bill. The rest of us are all jumping off the cliff. You're such a sissy!"

"Hey, did you notice the Thormplegorp Twins at the theater today? They refused to watch Eat My Shorts. What a couple of jam tarts, eh!"

Someone who only cares for appearance and lacks personality, they are into all the latest trends and shun people who are not the norm(unless they can use them for something). Jamtarts are often under 120 pounds and are usually seen in malls or popular bars wearing shirts too small for them, alot of pink, and have shoes less than 2 months old. They enjoy drinks with straws and standing around where they'll get the most attention.
They are also known to have poor spelling and grammar, and often talk without end about their dating habits or things they have recently purchased.

Jamtarts often don't know you're insulting them even when you do it to their face.

"Come on Ned, let's go to the bar and pick up some Jamtarts!"

"While walking through the mall, crowded with people, we happened upon a fashionable clothing store, completely infested with Jamtarts."